This invention relates to novel plastic compositions having useful physical properties and enhanced environmental degradability.
The advent of plastics has introduced improved methods of packaging goods. For example, polyethylene and polypropylene films, bags, and bottles, and polystyrene foam cups have the advantages of being chemically resistant, mechanically tough, light in weight, and inexpensive. However, the increasing use of plastics in packaging has led to the appearance of such materials in litter. While littered plastic articles are no more objectionable than littered articles of other materials, such as paper objects and metal cans, it has been suggested that the impact of plastic litter can be minimized by the development of plastic materials capable of undergoing chemical degradation upon exposure to the natural environment.
Several approaches to the enhancement of the environmental degradability of plastics have been suggested. These include: (1) the incorporation of particulate biodegradable materials such as starch as "fillers"; (2) the introduction of photodegradation-sensitizing groups into the molecular structure of a polymer by copolymerization of a common monomer with a second monomer processing such groups; and, (3) the incorporation of small amounts of selected additives which accelerate oxidative and/or photo-oxidative degradation. The last approach is particularly attractive for the following reasons. First, the physical properties of the additive-containing composition are extremely similar to those of the base polymer. Second, existing compounding and fabrication processes and equipment can be utilized in the manufacture of finished products; hence, the cost of the finished product should be relatively low. Third, the sensitivity of the composition to environmental degradation can be controlled by proper selection of the type and concentration of additive(s).
The enhancement of the rate of environmental deterioration of plastics through the use of oxidation-promoting additives is known in the prior art. For example, the preparation of degradable polyolefin films containing certain organic derivatives of transition metals is described in U.S. Pat. No. 3,454,510.
While transition metal compounds are effective accelerators of oxidative and photo-oxidative degradation processes, the use of such materials in the preparation of useful plastic compositions having enhanced degradability suffers from several limitations. The high reactivity of plastic compositions containing transition metal compounds leads to rapid degradation during high temperature extrusion and molding processes, with the result that such processes are difficult to control and tend to produce materials having poor mechanical and chemical properties. Some transition metal compounds impart undesired coloration to plastic compositions into which they are incorporated. Furthermore, the toxicity of some transition metal compounds limits their utility as additives in compositions intended for food packaging.
In accordance with this invention, the reactivity of an organic polymer toward oxidative and/or photo-oxidative degradation is significantly enhanced by the incorporation of at least one readily autoxidizable unsaturated wax.
The mechanism by which the addition of a readily autoxidizable unsaturated wax enhances the susceptibility of the base polymer to environmental degradation has not been established in detail. However, it is believed that thermal or photochemical autoxidation converts the additive to an intermediate such as a hydroperoxide which subsequently undergoes thermal or photochemical decomposition to free radicals; such radicals then initiate autoxidation of the polymer.